Phase transition behaviour and equilibrium phase relations in the fast-ion conductor system Na3PO4-Na2SO4

The phase transition behaviour and equilibrium phase relations in the fast-ion conductor system Na3PO4- Na2SO4 have been investigated using a combination of in situ XRD, neutron powder diffraction, and molecular dynamics simulation. The simulations predict that tetrahedrally-coordinated Na is relatively tightly bound within its interstitial site, whereas octahedrally-coordinated Na shows enhanced thermal vibration and often performs large amplitude excursions towards neighbouring interstitial sites. These observations are confirmed by structure refinements of the cubic (gamma) phase, which show mean-square displacement parameters for octahedral Na a factor of 5 larger than for tetrahedral Na. The solubility of Na2SO4 in the tetragonal (alpha) phase is very low (<<1%). Samples of the solid solution exist as a mixture of alpha and gamma phases, with the (alpha + gamma) --> gamma transition temperature decreasing linearly with increasing Na2SO4 content. Kinetic stabilisation of gamma to room temperature was achieved by substitution of 10% Na2SO4. The behaviour of samples containing less than 10% Na2SO4 is complicated by sluggish kinetics and the presence of two intermediate phases with triclinic and monoclinic symmetry (beta(t) and beta(m), respectively). These new phases are closely related to gamma, with the gamma --> beta transition being kinetically favoured over the gamma --> (alpha + gamma) transition on cooling.